Tektronix TDS3012B Manual de usuario

University of Portland - p. 1 of 9 - Oscilloscope - TDS3012B.docx

Oscilloscope Tutorial

Tektronix TDS3012B and TDS3012

Figure 1: Oscilloscope with a 10X probe connected to the Probe Comp Terminals

Introduction

The oscilloscope, which displays voltage versus time, is an indispensable tool for testing and

troubleshooting circuits and taking electrical measurements. This tutorial will explain how to set

up the oscilloscope and take basic measurements.

Getting Started

Do the following steps to set up the oscilloscope.

1. Turn on the oscilloscope by pressing the power button located in the lower left-hand

corner. Wait for scope to boot-up. When a message shows up on the screen, press the

MENU OFF button.

2. When the oscilloscope powers up, it uses the settings that were in place when it was last

powered down. To return the oscilloscope to the factory settings, press the Save/Recall

button. Then press the button under the screen labeled Recall Factory Setup. Next press

the button on the right of the screen labeled OK Confirm Factory Init.

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3. Connect a 10X high-impedance probe (see Figure 2) to the channel 1 BNC connector

which is labeled as CH 1. When connecting the probe, gently push the connector while

rotating it clockwise until it slides in, then turn it clockwise so that it snaps securely in

place. The 10X high-impedance probe should ONLY be connected to the oscilloscope,

not to any other equipment. (To remove the probe, turn it counter-clockwise to release,

then gently pull on the connector. Please do NOT pull on the cable.)

Figure 2: 10X High-Impedance Scope Probes (Two Different Models)

4. The alligator clip on the probe is the ground lead. In order to make sure that the probe is

working correctly, connect the ground lead to the lower connector of the Probe Comp

port as shown above in Figure 1.

5. If you gently push on the probe tip, a hook is exposed that can be connected to a wire.

Connect the probe tip to the upper Probe Comp terminal (see Figure 1). The Probe Comp

port generates square wave that oscillates between 0 and 5V.

6. Press the Autoset button, which selects certain settings automatically depending on the

input signal. The display should look similar to that in Figure 1.

7. Check that the oscilloscope is reading the Probe Comp port voltage correctly (see below

for details on how to read the voltage). If the voltage isn’t reading correctly, refer to the

section Probe Setup below.

8. Connect the probe tip to the voltage in the circuit that you want to measure. Connect the

alligator clip to the circuit ground.

Note that the ground lead of the oscilloscope is connected to earth ground internally by

the oscilloscope, so it should only be connected to ground in a circuit. If the ground lead

is connected anywhere else in a circuit, it will short that node to earth ground.

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Reading the Display

The oscilloscope generates a graph with volts on the vertical axis and time on the horizontal axis.

This section will explain how to read the display in order to measure time and voltage.

Measuring Voltage

The location on the screen that represents ground (or 0 Volts) is marked by an arrow on the left

side of the screen (see Figure 3). Note that the screen is divided into 8 divisions vertically,

where the divisions are marked by dotted lines on the screen. The number of volts that each

division represents (or volts/division) is displayed in the lower left part of the screen. To

measure a voltage, we can count the number of divisions above or below the ground level and

multiply by the volts/division value. For example, in Figure 3, the maximum voltage of the

square wave is 2.5 divisions above the ground level, so the voltage is 2.5 divisions * 2

volts/division = 5 Volts. If the voltage is below the ground level, then the voltage is negative.

Figure 3: Oscilloscope Screen

Measuring Time

Note that the screen is divided into 10 divisions horizontally, where the divisions are marked by

dotted lines on the screen. We can measure the time between two parts of a signal by counting

the number of divisions in the horizontal direction and multiplying by the number of seconds per

division. In Figure 3, for example, there are 2.5 divisions in one period of the wave, so the

period is 2.5 divisions * 400 µS = 1 mS.

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Adjusting the Display

Although the Autoset button automatically selects some of the settings for you depending on the

current input signal, it is often necessary to manually adjust the settings to get a useful display.

The oscilloscope has adjustments that allow you to set the vertical and horizontal scales

independently, and to slide the graph vertically and horizontally.

Adjusting the Vertical and Horizontal Scale

The vertical scale knob shown in Figure 4 adjusts the volts per division setting of the vertical

axis. This adjustment allows you to zoom in to observe small signals and zoom out for large

signals. Note that when you rotate the knob, the value of the volts/division is updated in the

lower left of the screen. In order to get an accurate voltage measurement, we need to set the

vertical scale so that the signal is as big as possible without going off the top and/or bottom of

the screen.

Figure 4: Vertical Scale Knob

The horizontal scale knob shown in Figure 5 adjusts the seconds per division setting of the

horizontal axis. This adjustment allows you to zoom the horizontal scale. Note that when you

rotate the knob, the value of the seconds/division is updated in the lower middle of the screen. In

order to get an accurate time measurement, we need to set the horizontal scale so that the display

shows one or two periods of the signal.

Figure 5: Horizontal Scale Knob

When the vertical position knob (Figure 6) is rotated, the signal slides up or down and the arrow

that marks the location of ground (0 Volts) is updated. The horizontal position knob (Figure 7)

slides the signal left or right.

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Figure 6: Vertical Position Knob Figure 7: Horizontal Position Knob

Coupling

The oscilloscope has three coupling modes which determine how the signal is processed before it

is displayed. In order to select the coupling mode, press the Vertical Menu button (see Figure 8),

then press the Coupling button under the screen (Figure 9), and then select the coupling mode

with one of the buttons on the right of the screen (Figure 9).

Figure 8: Vertical Menu Button Figure 9: Coupling Button

•DC: In DC coupling mode, no processing is performed on the signal before it is

displayed, and the display represents the signal with both DC and AC parts. This is the

default mode, and is used in most cases.

•AC: The AC mode is used for signals that have a small AC signal added to a large DC

component, and you are only interested in the AC signal. This mode is often used when

testing transistor amplifiers. In AC coupling mode, the DC component is blocked, and

only the AC part of the signal is displayed. This mode allows you to expand the vertical

scale to zoom in on the small AC signal, but keep in mind that the displayed voltage is

the actual voltage minus the DC offset, NOT the actual voltage.

•GND: In ground mode, the input signal is replaced by ground, which causes the

oscilloscope to draw a line on the screen at the ground level (0 Volts). This mode is used

to check where ground is on the screen. (There is also an arrow on the left side of the

screen that marks the ground level.)

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Triggering

The oscilloscope repeatedly draws the input voltage on the screen from left to right, then returns

to the left side and draws the signal again on top of the previous curve. Each time the

oscilloscope draws the signal, it is called a sweep. If the oscilloscope did not have a way to

arrange it so each sweep started at the same point in the period for periodic signals, the screen

would display multiple sweeps, which would interfere with taking measurements (see Figure 10).

Figure 10: Oscilloscope is not triggering properly

Triggering is a mechanism that causes the oscilloscope to pause the sweep at the left side of the

screen until the input voltage crosses a user-selected voltage called the trigger level. This causes

each sweep to start in the same point in the period for periodic signals, and leads to a stable

display.

If the input signal never crosses the trigger level, the oscilloscope will wait for about a second,

and then it will draw the sweep. In this case the display may appear to drift across the screen or

may have several sweeps superimposed on the screen at the same time. Either way, it is difficult

or impossible to make measurements if the oscilloscope is not triggering properly.

The trigger level knob controls the trigger level (see Figure 11). In order for the oscilloscope to

trigger properly, the trigger level must be set to a voltage that the input signal crosses.

Figure 11: Trigger Level Knob

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It is often difficult to trigger properly on noisy signals. Fortunately, the oscilloscope has a mode

where it passes the trigger signal through a lowpass filter (LPF), which eliminates high

frequency noise. (This LPF only affects the trigger signal, not the displayed signal.) To activate

this mode, press the Trigger Menu button, then Coupling, then HF Reject.

If it is impossible to trigger on a signal for some reason, another option is to temporarily freeze

the display by pressing the Run/Stop button, which will also cause “Stop” to be displayed in the

upper left-hand corner of the display. Press the Run/Stop button again to restart the oscilloscope.

Measurements using the Cursors

The cursors can be used to measure time differences or voltage levels on the oscilloscope.

Measuring Time Difference

As mentioned above, one way to measure the time difference is to multiply the number of

horizontal divisions by the seconds/division value. This method is often quick and easy, but it is

sometimes difficult to measure the number of divisions accurately. An alternative approach is to

use the vertical cursors, which are sometimes more accurate. Activate the vertical cursors by

pressing the Cursor button (see Figure 12), and then pressing the V Bars button (Figure 13).

Figure 12: Cursor Button Figure 13: V Bars Button

Turning the knob (see Figure 14) moves one of the cursors. To move the other cursor, press the

Select button and then turn the knob. The time difference between the cursors is displayed after

the Δ symbol in the top right of the screen (see Figure 13). The time measurement will be most

accurate if the horizontal scale is adjusted so that the time difference is as wide as possible.

Figure 14: Cursor Knob and Select Button

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Measuring Voltage

As mentioned above, one way to measure voltage is to multiply the number of vertical divisions

by the volts/division value. This method is often quick and easy, but it is sometimes difficult to

measure the number of divisions accurately. An alternative approach is to use the horizontal

cursors, which are sometimes more accurate. Activate the horizontal cursors by pressing the

cursor button (see Figure 15), and then pressing the H Bars button (Figure 16).

Figure 15: Cursor Button Figure 16: H Bars Button

Turning the knob (see Figure 17) moves one of the cursors and the voltage at the cursor level is

displayed in the upper right-hand corner of the screen after the “@” symbol. To move the other

cursor, press the Select button and then turn the knob. The voltage difference between the

cursors is displayed after the Δ symbol.

Figure 17: Cursor Knob and Select Button

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Automated Measurements

The oscilloscope can also measure various values automatically. To activate one or more of

these measurements, press the Measure button, Select Measrmnt, and then the button next to the

desired measurement. To see the full list of automated measurements, click on the More button.

To turn a measurement off, press the Measure button, Remove Measrmnt, and then Measurement

1 button (or the button for the measurement you wish to remove).

Depending on the vertical and horizontal scale settings, the automated measurements are not

always accurate, so it is good practice to double-check these measurements by multiplying the

number of divisions by the scale or by using the using the cursors.

Displaying Two Signals Simultaneously

The oscilloscope can display two signals at the same time. To add a second signal, connect a

10X probe to the CH 2 input and connect it to the signal that you want to display. Press the blue

CH 2 button to turn on the channel 2 trace on the oscilloscope display. Pressing the CH 2 button

also causes the Vertical Scale and Position knobs to control the channel 2 trace on the screen

instead of the channel 1 trace. If you want to adjust the vertical scale or position of Channel 1,

you first switch back to Channel 1 by pressing the CH 1 button, and then turn the Vertical Scale

or Position knob. The horizontal scale is always the same for both channels and can be adjusted

when either channel is active. You can turn one of the channels off by pressing the CH 1 or CH

2 button and then pressing the OFF button.

Probe Setup

There is a connection on most (but not all) 10X probes that tells the oscilloscope that a 10X

probe is connected. Since the 10X probe divides the input signal by 10X, if the oscilloscope

detects a 10X probe is connected, it will multiply the signal by 10 times to compensate. If the

Probe Comp signal (see Figure 1) does not appear to be a square wave from 0 to 5 Volts, check

to see if the oscilloscope has detected the 10X probe correctly by pressing the Vertical Menu

button, and then press the Probe Setup button. The value should be 10X if a 10X probe is

connected to the input (and 1X if a coax cable is connected).

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